Disclosures About Offsetting Assets And Liabilities (Financial Liabilities And Derivative Liabilities That Are Subject To Offset) (Details) (Parenthetical) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Fair value of securities pledged as collateral to counter party	[1]	$ 233,711	$ 80
U.S. Government Agency Securities And U.S. Government Securities And Corporate Fixed Income Securities [Member] | Fair Value Of Securities Pledged As Collateral [Member]
Offsetting Liabilities [Line Items]
Fair value of securities pledged as collateral to counter party		$ 522,400	$ 241,400

[1] Collateral pledged includes the fair value of securities pledged by our company to the counter party. These securities are included on the consolidated statements of financial condition unless we default. Collateral pledged by our company to the counter party includes U.S. government agency securities, U.S. government securities, and corporate fixed income securities with market values of $522.4 million and $241.4 million at September 30, 2018 and December 31, 2017, respectively.